This research program has developed a unique classification system for Speech Sound Disorders (SSD) termed the Speech Disorders Classification System (SDCS). Research goals are to understand the causal pathways that lead to each of eight subtypes of SSD so that clinicians can select the appropriate intervention approach for each child, and researchers can conduct studies that should lead to prevention of some subtypes of SSD. The final phase of this project will complete all goals by obtaining new information in three study series. Diagnostic Accuracy Studies. Using a unique risk factor classification system for SSD, including genomic and environmental variables, we will document speech error differences associated with how children encode speech signals, how they store and access linguistic representations of speech, and how they transcode representations into the movement gestures for speech production. Prevalence Studies. Our research indicates significant differences in the prevalence of subtypes of SSD and in the rates and types of normalization outcomes. The proposed research will estimate both clinical prevalence rates and population prevalence rates for six subtypes of SSD. Speech Processing Studies. Our studies have motivated questions about speech processing differences and similarities in speakers from different causal backgrounds. In addition to studies of speakers with speech delay and motor speech disorders of unknown origin, our questions will be informed by assessment data from speakers with Down syndrome, 22q11.2 deletion (Velocardiofacial) syndrome, Galactosemia, Joubert syndrome, and Rolandic Epilepsy. A number of hypotheses about speech processing differences between and within study groups will be addressed, including a focus on support for a speech sound disorder termed Childhood Apraxia of Speech and a placeholder motor speech disorder we term Motor Speech Disorder-Not Otherwise Specified. Relevance The high prevalence of SSD at 6 years of age (3.8%: Shriberg, Tomblin, and McSweeny, 1999), persisting to 8 years of age (3.6%: Wren et al., 2009), places SSD among the most frequently occurring childhood disorders warranting public health resources for research in prevention, assessment, and treatment. SSD puts a child at risk for literacy development, reduced peer acceptance, and limitations in vocational options. Findings from this research should have substantial impact on research and clinical practice for this significant childhood communicative disorder.